Bionomics aspects of major arbovirus vectors, Aedes aegypti and Aedes albopictus in different ecoclimatic environments of Cameroon

Mosquitoes are more than just a summer nuisance; for millions of people, they are the primary delivery system for some of the world's most dangerous viral diseases. In many parts of the globe, including Africa, viruses like dengue, Zika, and chikungunya circulate silently, carried by specific types of mosquitoes that thrive in human settlements. To stop these diseases, health officials must understand the daily lives of these insects: where they hide when they are not biting, when they are most active, and how they interact with the changing environment. This knowledge is the foundation of any effective control strategy. Without it, efforts to reduce mosquito populations can be misdirected, wasting resources and leaving communities vulnerable. Two species, Aedes aegypti and Aedes albopictus, are the main culprits behind these outbreaks. While Aedes aegypti is native to Africa and has lived alongside humans for centuries, Aedes albopictus is an invasive species from Asia that has spread rapidly across the continent in recent decades. Understanding how these two rivals coexist, compete, and behave in different climates is essential for predicting where the next outbreak might strike.

In Cameroon, a country with diverse landscapes ranging from dry northern savannas to humid southern rainforests, scientists set out to map the current reality of these mosquitoes. For years, there was a gap in knowledge regarding exactly where these insects lived, how they rested, and whether they were already carrying viruses. A team of researchers from the Centre for Research in Infectious Diseases in Cameroon, working alongside colleagues from the London School of Hygiene & Tropical Medicine, conducted a massive survey across eight locations in the country. They did not just look for mosquitoes; they tracked their entire life cycle. The team collected immature mosquitoes, known as larvae, from water-filled containers like old tires, discarded bottles, and natural tree holes. They brought these larvae back to a laboratory, raised them until they became adults, and then carefully identified each one. This process allowed them to build a precise picture of which species were present in which neighborhoods, from the bustling cities of Douala and Yaoundé to the rural edges of the forest.

The results painted a clear picture of a shifting landscape. The researchers identified over 17,000 adult mosquitoes from the larvae they collected. They found that Aedes albopictus had become the dominant species in most of the southern part of the country, making up just over half of all the mosquitoes found. In contrast, Aedes aegypti was present in every single location they surveyed, but it was often outnumbered by its invasive rival in the south. A significant discovery was the first confirmation of Aedes albopictus in the town of Meiganga, marking a new step in its expansion. However, the balance of power was not uniform. In the city of Douala, Aedes aegypti remained the most common species, while in the northern regions, the invasive species struggled to establish itself, likely due to the drier, hotter climate. This suggests that while the invasive mosquito is winning the battle for territory in many areas, local environmental conditions still dictate where it can survive.

Beyond simply counting heads, the team wanted to know where these mosquitoes went to rest during the day. This is a critical detail because if mosquitoes hide inside houses, spraying indoor insecticides works well. If they hide outside, different strategies are needed. The researchers spent hours searching for resting mosquitoes in and around homes, looking under furniture, in vegetation, and inside discarded tires. They found that both species in Cameroon are overwhelmingly "exophilic," meaning they prefer to rest outdoors rather than inside human dwellings. Only a tiny fraction of the mosquitoes were found inside houses. Instead, they were found in the open air. Aedes aegypti showed a strong preference for resting in used tires, which provided shade and protection. Aedes albopictus was more flexible, resting in a wide variety of places including natural vegetation, banana plants, and mixed areas where different types of plants grew together. This behavior highlights a key difference: the native mosquito clings to man-made objects, while the invasive one thrives in the greenery surrounding homes.

The timing of their activity also revealed important patterns. Using specialized traps baited with the scent of guinea pigs, the researchers monitored when the mosquitoes were most eager to bite. They found that both species are active during the day, from early morning until evening. However, their peak activity was not in the cool of the morning or the heat of noon, but rather in the afternoon. For both species, the most intense biting activity occurred between 1 p.m. and 6 p.m. This unimodal pattern, with one major peak in the afternoon, was consistent across most locations, though some specific neighborhoods showed a secondary, smaller peak in the morning. The study also noted that the abundance of these mosquitoes changed with the seasons. Aedes aegypti numbers surged during the rainy season, likely because the rain filled up more artificial containers like tires and buckets, creating more breeding sites. Conversely, Aedes albopictus was often more abundant during the dry season, perhaps because the vegetation it prefers for resting remains stable while the rain washes away other potential hiding spots.

Perhaps the most urgent finding concerned the presence of viruses. The researchers tested thousands of female mosquitoes to see if they were carrying dengue, chikungunya, or Zika viruses. They found evidence of dengue virus in one group of Aedes aegypti mosquitoes collected in Douala. Specifically, the virus was identified as serotype 3, a common strain of dengue. This confirmed that the virus was actively circulating in the local mosquito population, even though no major outbreak was reported at the time of the study. No mosquitoes were found carrying chikungunya or Zika viruses, and none of the Aedes albopictus mosquitoes tested positive for any of the three viruses. This suggests that while the invasive mosquito is a formidable competitor for space, the native Aedes aegypti remains the primary driver of dengue transmission in this region.

These findings provide a vital roadmap for public health officials in Cameroon. The data shows that controlling these mosquitoes requires a strategy that looks outward, focusing on the vegetation and discarded tires surrounding homes rather than just the interiors of buildings. It also highlights the need to monitor the expanding range of the invasive Aedes albopictus, which could eventually change the dynamics of disease transmission if it becomes more efficient at carrying viruses. The detection of dengue virus in the local mosquito population serves as a warning that the conditions are ripe for an outbreak. By understanding exactly where these insects live, when they bite, and what they carry, health authorities can design targeted interventions that protect communities more effectively. The study underscores that in the fight against mosquito-borne diseases, the details of the insect's daily life are just as important as the virus it carries.